The present invention relates to an engine brake running control for an automatic transmission when the automatic transmission operates in an automatic drive range, viz., a "D" range.
An automatic transmission of the E4N71B type is known. This automatic transmission is manufactured by Nissan Motor Company Limited in Japan and described in a publication "Nissan Automatic Transmission L4N71B Type, E4N71B Type, Service Manual 1982 (A261C04)" issued by Nissan Motor Company Limited on November in 1982. This known transmission comprises a downshift solenoid, a shift switch, an idle switch, a vehicle speed sensor, a brake switch, and a control unit. As described on page 12 of the above-mentioned publication, the downshift solenoid is rendered ON when the shift switch indicates that the third gear position is established in D range, the vehicle speed sensor detects a vehicle speed falling in a predetermined range from 30 km/h to 50 km/h, the brake switch is rendered ON, and the idle contacts of the throttle switch are rendered ON. According to this known downshift control, whenever the above mentioned conditions are met, a downshift is made to the adjacent one low gear position, and further downshift will not be made. Thus, if more effective engine brake is needed, it is necessary to shift a manual selector to a manual select range position, such as "2" range or "1" range.
The Applicant has proposed in the above-mentioned U.S. patent application Ser. No. 07/342,144 filed on Apr. 24, 1989 how to determine a new gear position which an automatic transmission should be downshifted to and conditioned for an engine brake running when the automatic transmission operates in a D range (an automatic drive range). According to this previous proposal, the new gear position is determined in response to a deceleration which the automotive vehicle is subject to after a brake pedal has been depressed. More specifically, predetermined deceleration values are provided for different gear positions and an appropriate one of them which corresponds to the present gear position which the automatic transmission is conditioned in is selected and compared with the actual deceleration detected. In response to the result of this comparison, a new gear position lower than the present gear position is determined.
An object of the present invention is to improve the previously proposed engine brake running control such that a new gear position which the automatic transmission should be downshifted to and conditioned for engine brake running is determined in a manner to compromise different requirements.